Morphing user interfaces will be an important design consideration for the next generation of portable electronic devices. A “morphing user interface” is an interface whose appearance changes as the device's use changes (e.g., from a phone to a camera, camera to music player, music player to phone, etc.). This input interface is simpler and more intuitive to use since only the context-relevant functions are shown at any given time, with the interface elements that are not related to the current context being inactive and hidden. The concept of a morphing user interface also preferably includes the total lack of feedback to the user when the user may contact those user interface elements which are hidden and inactive in the current user interface context.
Traditionally, mechanical dome switches have been used to provide haptic (tactile) feedback when users press a key. However, dome switches do not function well with morphing graphic user interfaces; therefore, haptics or active feedback becomes a critical enabler. Rotary or linear vibration motors can provide tactile feedback of sorts with optimized driving algorithms, but their buzz-like vibration profiles are very different from a dome switch's sharp mechanical click. On the other hand, piezoelectric actuators can produce a much more realistic click sensation, providing the perception that the user has pressed a real, physical key. This realistic click can be applied to individual surfaces, creating a more “local” response (like a dome switch) as opposed to the “global” response of vibration motors that shake the entire device.
This localized tactile feedback, which can be alternatively termed “localized haptics”, sends tactile feedback to a user by means of movement of a portion of a handheld device, or portions of its surfaces. Locally actuated touch screen and navigation keys are two examples of localized haptics. In the case of a cell phone, the feedback can be limited to a navigation key, a touch screen, or buttons on holding surfaces of the phone, e.g., side stripes.
One type of haptic feedback is described in U.S. Pat. No. 6,710,518. An electromechanical transducer produces an impulse of mechanical energy that propagates through a mounting boss to the entire device. This mechanism is suitable for providing a “call alert” which vibrates the entire device, but does not allow for selective feedback to individual input locations (keys, buttons, arrows, etc).
U.S. Patent Publications 2006/0050059 and 2006/0052143 present another type of haptic feedback. One or more piezoelectric actuators are placed, typically at the corners, under an input device that needs to be actuated (e.g., a keypad or a touch-sensitive display). When a voltage is applied, the piezoelectric actuators deform, either pushing or pulling the entire input device in a given direction. As a result of this movement, the device gives a tactile response to the user's hand or finger operating at the input device. The most widely used piezoelectric actuators for this purpose are either unimorph or bimorph actuators (also referred to as “benders”). Unimorph actuators are made of a single piezoelectric ceramic element bonded to a metal shim, whereas bimorph actuators comprise a metal shim bonded between two piezoelectric ceramic elements. The unimorph actuator's bending motion comes from the tendency of either in-plane shrinkage or expansion of the piezoelectric ceramic element under applied electric field against the mechanical constraint from the metal shim. In the case of a bimorph actuator, the two piezoelectric ceramic elements are driven such that one shrinks while the other expands, both in their respective planes, causing the bending motion. A typical placement of the benders is to anchor the edge of a circular bender, or both ends of a stripe bender, on a base structure. The center of a circular bender, or the middle of a stripe bender which has the maximum displacement, is usually used to drive a mechanical load, as illustrated in both U.S. Patent Publications 2006/0050059 and 2006/0052143. It is worth noting that stand-alone piezoelectric ceramics cannot generate these relatively high displacements; rather it is the bonded structure of the piezoelectric ceramic element(s) and metal shim that makes such high displacement possible.
It is challenging to optimize piezoelectric actuator drive circuits for handheld devices. The circuit must be able to drive significant capacitive loads (e.g., 100 nanofarads) to peak voltages of 100 or more volts, with controlled rises and falls in voltage and time using low supply voltages (e.g., 3V-5V).
Known circuits for driving a piezoelectric actuator (e.g., one or more piezoelectric elements) may utilize short low voltage control pulses to control and shape the drive signal of a piezoelectric actuator such that the output to the actuator approximates a voltage sinusoid. However, such solutions in the prior art are typically designed for driving a piezoelectric element bidirectionally (i.e. from a large negative to large positive voltage). As such, these circuits are non-optimal for generating a positive voltage waveform as required to simulate a key click tactile feel. Not only are they overly complex and costly to implement, but if driven at a high enough voltage, they can even de-polarize the piezoceramic element and thus render piezoelectric unimorph actuators ineffective. In addition, solutions have been proposed in which arbitrary predefined waveforms such as saw-tooth, sine, half sine, and pulse, are played to create specific haptics effects based on a user input; however, such solutions teach no method of generating such signals and do not teach a suitable method of generating such signals at high enough voltages to drive a piezoelectric unimorph actuator.
Accordingly, it is desirable to provide electronic devices having click-like tactile feedback provided by low cost, thin piezoelectric devices, driven by a very simple and low-cost, yet highly flexible, drive circuit. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.